Femtosecond optical vortices, combining the spatial characteristics of the helical phase and the annular beam with femtosecond time duration, may open up a variety of fascinating applications, ranging from femtosecond micro-nano manipulation to vortex strong-field physics. Each photon in a vortex beam carries an orbit angular momentum of lh, where l is the topological charge number.
The traditional phase- or diffraction element-based (spiral phase plate, hologram grating, etc.) vortex generation techniques are only suitable for CW and picosecond lasers with a narrow spectral bandwidth, where the effect of dispersion may be ignored. As femtosecond vortex has wide spectral bandwidth, the traditional phase- or diffraction element-based vortex generation techniques are intrinsically limited by dispersion, which will result in unclean femtosecond vortex generation. With the traditional vortex generation techniques, the central node of the femtosecond vortex is not clean. In other words, the femtosecond vortex has a low spatial intensity contrast. Meanwhile, high spatial intensity contrast is of great importance for the femtosecond vortex application, especially in the strong-field physics field.